In contact printing, a printing agent is applied onto a printing surface, a sheet material is impressed against the printing surface, and the sheet material is then separated from the printing surface. The processes and apparatus for such contact printing can take many forms. For example, the printing surface may be formed on a flat plate or block, on a cylindrical shell or roller, on a removable plate mounted on a shell or roller, or in any other required or convenient form. The sheet material may be processed as a continuous web, as individual sheets, as a web already partially separated into individual sheets, such as by perforation, as folded individual sheets or webs, and so on. The printing agent may be an ink, a dye, an adhesive, or any other material having the fluid properties necessary for the particular printing application. The printing agent may be applied onto the printing surface by means of an applicator, such as a roller, or may be extruded through a porous printing plate onto the printing surface.
In a contact printing process, the printing agent may accumulate on the printing surface and form a hard protuberance or a mass that may detach from the printing surface and contaminate the process. Also, the printing agent may adhere to both the printing surface and the sheet material with sufficient strength to cause the rupture or distortion of the sheet material when it is separated from the printing surface. For example, the avoidance of ruptures or distortion is especially important when printing a relatively aggressive adhesive onto a relatively thin and conformable film, such as in the manufacture of a film for wrapping food or food containers, but may be especially difficult to achieve.
It is preferable to prevent or minimize the strong adhesion of the printing agent to the printing surface, rather than to add process steps or equipment in an attempt to compensate for its occurrence. For example, a release agent such as an oil may be externally applied to a printing roller by means of an applicator roller, a brush, or a non-contact applicator. Such an approach is limited in its usefulness by practical considerations such as the requirement for space immediately adjacent the printing roller 16 and the difficulty inherent in attempting to apply the release agent in equal amounts per unit area on specific portions of the printing surface corresponding to where the printing agent will be applied, in order to minimize the usage of the release agent and the possibility of contamination of the process by excess release agent.
Also, the consistent external application of a release agent in pure form at a relatively low rate is often difficult to achieve. An emulsion of a release agent may be used to facilitate the external application, but the emulsifier often has undesirable properties relative to the process and the finished product. Therefore, it may be necessary to volatilize a part of the emulsion immediately after its application to the printing surface, for example, through the application of heat energy. However, the temperature required for volatilization may be excessive for the material of which the printing surface is made, which is often selected on the basis of its ease of machining.
In addition, printing processes in which the printing agent is extruded through a porous printing plate present additional difficulties with respect to the prevention of the adhesion of the printing agent to the printing surface. These difficulties arise from the direct application of the printing agent to the printing surface and the resultant effective preclusion of the use of an externally applied release agent, because of the impracticality of applying the release agent onto the printing surface beneath the printing agent.
An alternative approach to the prevention of the adhesion of the printing agent to the printing surface is to use a printing plate impregnated with a fixed quantity of a release agent that is depleted over a number of cycles of the process. In this approach, the progressive depletion of the release agent may lead to a progressive reduction in effectiveness. A similar approach is to make the printing surface of a material such as silicone rubber or a urethane having good release properties. However, a printing plate fabricated of such a material often lacks the desired durability. Another approach is to apply a more durable release agent, which may be renewed when worn or degraded, to the printing surface. Examples of such durable release agents are various plasma coatings, polymer coatings, and films or sheets of such materials, which may be affixed to the printing surface. However, the use of such durable materials requires the continuing monitoring, maintenance, and replacement of the materials in order to maintain their effectiveness. Also, damage to such materials or their structural failure may result in the contamination of the process.
Another alternative approach to the prevention of the adhesion of the printing agent to the printing surface is to apply a low surface energy coating to the printing surface. For example, silicone-based and fluoropolymer-based coatings may have the desired release properties. However, some such low surface energy coatings lack sufficient durability for practical use in contact printing processes. Also, the curing temperatures required for the proper application of some of these coatings exceeds the temperatures at which creep or the failure may occur in the materials of which the printing plates are made. For example, it may not be practical to apply a fluoropolymer having a curing temperature of approximately 400 degrees C. to a structural material having a creep temperature of approximately 110 degrees C. and a failure temperature of approximately 200 degrees C.
Yet another approach is to maintain a process condition in which the printing agent will not strongly adhere to the printing surface. For example, some adhesives can be prevented from strongly adhering to a surface by maintaining that surface at a sufficiently high temperature. However, the required high temperature may be excessive for the sheet material being impressed in a contact printing process. In addition, at the required temperature, the adhesive may flow onto other surfaces where its presence is problematic. As another example, an adhesive may be prevented from adhering to a surface by chilling that surface to a temperature at which atmospheric moisture condenses and forms a layer of water on the surface. However, the presence of water in its liquid state is often problematic. Also, the rates of condensation and of the accumulation of water on the surface depends on the relative humidity, the rate at which the sheet material removes the water from the surface, and other factors. Variations in these factors can lead to the accumulation of ice on the surface, which often is unacceptable. In addition, the chilling of a surface to a condensation temperature typically requires a channel near the surface for the circulation of a chilling agent, which limits the configuration of the printing plate.
Therefore, a need exists for a contact printing process and apparatus in which the adhesion to a printing surface of a printing agent and a sheet material onto which it is printed can be prevented, without an external application of a release agent, a progressive depletion of a fixed quantity of a release agent, a non-durable printing surface, a source of process contamination in the form of a durable release agent, or an extreme process condition.